A new lithium-ion battery made entirely of solid material—no liquids or gels—cannot ignite, even at very high temperatures.

In most conventional batteries, like those used in phones and computers, the positive and negative poles—the two electrodes—are made of solid conductive compounds. Charges move between these electrodes in a liquid or gel electrolyte.

If you charge such a battery improperly (overcharging) or leave it sitting out in the sun, the liquid can ignite or the gel can swell up.

This is not the case with solid-state batteries, which are currently in development in research laboratories worldwide. Both the electrodes and the intermediary electrolyte are made of solid material.

“Solid electrolytes do not catch fire even when heated to high temperatures or exposed to the air,” explains Jennifer Rupp, a professor of electrochemical materials at ETH Zurich, who leads the team developing the new battery technology.

One of the challenges in developing solid-state batteries is to connect the electrodes and electrolyte in such a way that the charges can circulate between them with as little resistance as possible. Rupp and her team are testing an improved electrode-electrolyte interface.

In the laboratory, they constructed a sandwich-like battery featuring a layer of lithium-containing compound (lithium garnet), which acts as a solid electrolyte between the two electrodes. Lithium garnet is one of the materials with the highest known conductivity for lithium ions.

“During production, we made sure that the solid electrolyte layer obtained a porous surface,” says Jan van den Broek, a master’s student in Rupp’s group and one of the authors of the study published in Advanced Energy Materials.

Really high temperatures

The researchers then applied the material of the negative pole in a viscous form, allowing it to seep into the pores. Finally, the scientists tempered the battery at 100 degrees Celsius.

“With a liquid or gel electrolyte, it would never be possible to heat a battery to such high temperatures,” says van den Broek.

Thanks to the trick with the pores, the researchers were able to significantly enlarge the contact area between the negative pole and the solid electrolyte, which ultimately means that the battery can be charged faster.

Batteries produced like this could theoretically operate at a normal ambient temperature, says Semih Afyon, a former research scientist in Rupp’s group and now a professor at the Izmir Institute of Technology in Turkey. But they work best at 95 degrees Celsius and above at the current development state. “The lithium ions can then move around better in the battery,” says Afyon.

This characteristic could be put to use in battery storage power plants, which store excess energy and deliver it later as needed.

“Today, the waste heat that results from many industrial processes vanishes unused,” says Afyon. “By coupling battery power plants with industrial facilities, you could use the waste heat to operate the storage power plant at optimal temperatures.”

New thin-film batteries

“Many of today’s solid-state battery research projects focus on improving the electrolytes,” says Afyon. However, there are few studies such as this one, in which the scientists assembled an entire solid-state battery—using methods also used in industrial production—and tested it.

“In this work we have for the first time built a whole lithium-ion battery with a solid lithium garnet electrolyte and a solid minus pole made of an oxide-based material. Thus, we have shown that it is possible to build whole batteries based on lithium garnet,” says Rupp.

Thanks to this solid electrolyte one can not only operate batteries at higher temperatures, but also build thin-film batteries, that can even be directly placed on silicon chips.